Interrogating the Record of Archean Tectonics from Metamorphism in the Minnesota River Valley, USA

If and how plate tectonics operated in the Archean eon remains a controversial topic (Brown et al., 2020). Southernmost of the Superior Province, the Minnesota River Valley (MRV) gneiss terrane is comprised of multiple tectonic "blocks" with diverse igneous and metamorphic rocks dating back to over 3.5 Ga (Schmitz et al. 2018). Therefore, its geologic history offers a scope to understand the tectonic processes that drove the evolution of early continents. This study focuses on the granulite-facies Montevideo and amphibolite-facies Morton blocks of MRV, which are separated by the east-trending Yellow Medicine shear zone. We provide zircon and monazite U-Pb dates for ~20 gneiss samples from the two blocks and use major element diffusion chronometry of garnet to estimate the timing and duration of metamorphism. Inverse thermobarometry calculations and phase-equilibrium modelling yield P-T estimates for peak metamorphism. Together, these P-T-t data will be used to determine the tectonothermal environment(s) of Archean metamorphism in the two blocks and whether significant lateral translation of the blocks—akin to modern plate tectonics—might have been responsible for their juxtaposition.